The invention relates generally to protective circuits for a regulated direct-current power supply, and more particularly to protective circuits incorporated in power supplies installed in a missile launcher.
Many forms of military aircraft are provided with guided missile launchers which are mounted on the wing tips or attached to the fuselage. The launcher is permanently secured to its aircraft mounting and is coupled by an umbilical cable to the missile. A connection between the cable and the missile is effected by a releasable coupler such that when the missile is fired and the missile slides off the launcher, the coupler is automatically disengaged to free the missile.
In one known form of launcher, the launcher includes two regulated direct-current power supplies, both of which are operated by an alternating-current generator in the aircraft. One power supply delivers 25 volts with a 3-amp maximum current through the cable to circuits contained in the missile, while the other delivers 175 volts with a 0.2 amp maximum current.
In some instances, because of a defective cable coupler, a decoupling action fails to take place when the missile is fired, as a consequence of which the cable which remains linked to the launcher is ruptured by the departing missile. Such cable rupture may result in a short circuit across one or both of the power supplies. The power supplies in the launcher are potted and shorts thereacross result in overheating of the supplies and the destruction thereof. A short may also occur prior to launching because of a defect in the missile circuits coupled by the cable to the supplies, with the same damaging consequences.
The obvious solution to the short-circuit problem is to provide a circuit-breaking relay in the output of each regulated power supply. The relay is connected across a resistor in series with the output of the supply such that when a short occurs for any reason, the resultant high voltage drop across the resistor activates the relay to open the relay switch contacts to disconnect the supply from the load, thereby preventing the short from damaging the supply.
The serious objection to this obvious protective arrangement is that in the absence of a short or an excessive load having an equivalent effect, the voltage drop across the series resistor in normal operation varies with changes in the load imposed on the regulated power supply by the associated missile circuits; hence the voltage regulation of the supply is impaired.